The present invention generally relates to methods of crystal growth on a semiconductor material. More specifically, the present invention relates to crystal growth methods for a nitride semiconductor, such as a gallium nitride based semiconductor and a methods for forming semiconductor devices employing crystal growth methods of the present invention capable of fabricating a variety of semiconductor devices including, for example, a semiconductor light emitting device, such as a semiconductor light emitting diode, a semiconductor laser, a semiconductor transistor device or the like.
In general, known vapor-phase growth techniques for a nitride semiconductor, such as a gallium nitride based compound semiconductor, can be problematic as it is difficult to obtain a substrate being lattice matched with a nitride semiconductor or a substrate having a low density of dislocations. To solve such a problem, there has been known a technique of depositing a low temperature buffer layer made from AlN or AlxGa1-xN (0≦x<1) at a low temperature of 900° C. or less on a surface of a substrate made from sapphire or the like, and then growing a gallium nitride based compound semiconductor thereon, thereby reducing dislocations due to lattice mismatching between the substrate and the compound semiconductor. Such a technique has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. By using such a technique, it is possible to obtain a gallium nitride based compound semiconductor with improved crystallinity and morphology.
Another technique of obtaining high quality crystal at a low density of dislocations has been disclosed, for example, in Japanese Patent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This method involves depositing a first gallium nitride based compound semiconductor layer, forming a protective film made from a material capable of inhibiting growth of a gallium nitride based compound semiconductor, such as silicon oxide or silicon nitride, in such a manner as to selectively cover the first gallium nitride based compound semiconductor, and growing a second gallium nitride based compound semiconductor in an in-plane direction (lateral direction) from regions, not covered with the protective film, of the first gallium nitride based compound nitride layer, thereby preventing propagation of through-dislocations extending in the direction perpendicular to the interface of the substrate.
A further technique of reducing a density of through-dislocations has been disclosed, for example, in MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999). This method involves growing a first gallium nitride based compound semiconductor, selectively removing the thus formed semiconductor film by using a reactive ion etching (hereinafter, referred to as “RIE”) system, and selectively growing a second gallium nitride based compound semiconductor from the remaining crystal in the growth apparatus. According to this method, it is possible to obtain a crystal film having a density of dislocations, which is reduced to about 106/cm2, and hence to realize a high life semiconductor laser using the crystal film formed according to this method.
FIGS. 8A to 8D are sectional views showing steps of one related art crystal growth method for a gallium nitride based compound semiconductor. Referring to FIG. 8A, after a low temperature buffer layer is formed on a C-plane 101 of a sapphire substrate 100, the supply of trimethyl gallium is stopped while the supply of ammonia is continued, with a result that grains each having a size on order of several tens of nanometers, which are nuclei for formation of a gallium nitride layer 102, are formed. Referring to FIG. 8B, as the supply of trimethyl gallium begins again, crystal growth occurs from the grains, to form island crystal regions each laterally extending on the C-plane 101. When the crystal growth proceeds at a crystal growth rate of 4 μm/h (which crystal growth rate is a value converted into a crystal growth rate in film-like crystal growth on a plane), boundaries of the island crystal regions are bonded to each other as shown in FIG. 8C, and further, a gallium nitride layer 102 formed by bonding the boundaries of the island crystal regions to each other becomes thick as shown in FIG. 8D, whereby a desired crystal layer is formed on the sapphire substrate 100.
FIGS. 9A to 9D are sectional views, similar to those of FIGS. 8A to 8D, showing steps of another related art crystal growth method for a gallium nitride based compound semiconductor. In this example, as compared with the example shown in FIGS. 8A to 8D, the crystal growth rate (which is converted into a crystal growth rate in film-like crystal growth on a plane) is set to a value being as low as about 1 μm/h. Referring to FIG. 9A, after a low temperature buffer layer is formed on a C-plane 111 of a sapphire substrate 110, like the example shown in FIGS. 8A to 8D, grains each having a size on order of several tens of nanometers, which are nuclei for formation of a gallium nitride layer 112, are formed by stopping the supply of ammonia while continuing the supply of trimethyl gallium. Referring to FIG. 9B, as the supply of trimethyl gallium begins again, crystal growth occurs from the grains, to form island crystal regions each laterally extending on the C-plane 111. When the crystal growth proceeds at a crystal growth rate of 1 μm/h (which crystal growth rate is a value converted into a crystal growth rate in film-like crystal growth on a plane), boundaries of the island crystal regions are bonded to each other as shown in FIG. 9C, and further, a gallium nitride layer 112 formed by bonding the boundaries of the island crystal regions to each other becomes thick as shown in FIG. 9D. In this crystal growth, since the crystal growth rate is lower than that in the crystal growth shown in FIGS. 8A to 8D, lateral crystal growth becomes predominant growth, with a result that the density of dislocations becomes smaller than that in the crystal growth shown in FIGS. 8A to 8D.
FIGS. 10A to 10E are sectional views showing steps of a further related art crystal growth method for a gallium nitride based compound semiconductor, which method is intended to reduce dislocations by selective growth. Referring to FIG. 10A, a gallium nitride layer 122 is formed on a sapphire substrate 120, a silicon oxide film 123 is formed as an anti-surfactant film on the gallium nitride layer 122, and opening portions 124 are formed in the silicon oxide film 123. Referring to FIG. 10B, island crystal regions for forming a gallium nitride layer 125 are formed in the opening portions 124 by selective growth. Referring to FIGS. 10C to 10E, as crystal growth proceeds, boundaries of the island crystal regions are bonded to each other, whereby a gallium nitride layer 102 formed by bonding the boundaries of the island crystal regions to each other is formed to a desired thickness.
In the above-described technique using a low temperature buffer layer, as shown in FIGS. 8A to 8D and FIGS. 9A to 9D, since crystal growth nuclei formed by the low temperature buffer undergo pseudo two-dimensional growth, the density of dislocations such as screw dislocations 103 and 113 in lateral growth portions is reduced; however, edge dislocations 104 and 114 occur at portions where boundaries of island crystal regions are bonded to each other. As a result, in the case of using only such a technique, it is believed that a reduction in density of through-dislocations to a value in a range of less than about 109/cm2 cannot be obtained. On the other hand, in the technique of selectively forming a protective film on a first gallium nitride based compound semiconductor and re-growing a second gallium nitride based compound semiconductor or in the technique of selectively removing a first gallium nitride based compound semiconductor by RIE or the like and re-growing a second gallium nitride based compound semiconductor, as shown in FIGS. 10A to 10E, the density of dislocations in lateral growth regions becomes low; however, through-dislocations 126 occur at portions at which lateral growth regions are bonded to each other, with a result that it is difficult to realize a crystal layer having a low density of dislocations.
A technique of reducing a density of dislocations by supplying a silicon material as an anti-surfactant at the time of growth of a gallium nitride based compound semiconductor film has been known, for example, in Journal of Crystal Growth 205, 245 (1999). However, even in this technique, in the step that island crystal regions, each having a three-dimensional shape, undergo pseudo two-dimensional growth, to be laterally grown and thereby bonded to each other, dislocations occur at portions at which boundaries of the island crystal regions are bonded to each other.
A method of producing a group III-V compound semiconductor has been disclosed in Japanese Patent Laid-open No. Hei 9-97921, wherein after a buffer layer is formed, a GaN layer is formed at a growth rate of 1000 Å/min and then a non-doped GaN layer is formed at a growth rate of 200 Å/min. According to this method, a luminous efficiency can be enhanced by forming such a low rate growth layer. This invention is advantageous in enhancing quality of crystal by forming the low rate growth layer; however, this invention does not describe a method of suppressing through-dislocations, edge dislocations and screw dislocations from a substrate, and therefore, requires reduction in density of dislocations yet.
As described above, known crystal growth techniques for a nitride semiconductor have a limitation in reducing a density of dislocations insofar as the technique is singly used, and is disadvantageous in that the performance and service life of a semiconductor device formed on a semiconductor layer produced according to known techniques are degraded.
A need, therefore, exists to develop an improved method for crystal growth of a nitride semiconductor at a reduced density of dislocations that can be employed during the manufacture of semiconductor devices in a variety of suitable applications.